If you're the skeptical type, you might raise an eyebrow when you hear that a
particular Viking ship was built in the year 819. How could anyone determine
the age of such an aged object so precisely, especially when there are
absolutely no records to verify the date?

Well, tree-ring dating, or dendrochronology, can be this precise, and
even more so. Dendrochronologists showed that an ancient wooden road uncovered in
southwestern England not only was built in 3806 B.C., but that the trees
used for the road were chopped down in the winter of that year (the winter of
3807-3806). The science can also reveal the origin of old pieces of wood. In
one case, archeologists determined not only the age of a Viking ship found in
Denmark, they also learned that it was built in Ireland.

Tree rings can reveal more than just a tree's age.

The basis of dendrochronology lies in a tree's unique pattern of growth rings.
The rings form a kind of fingerprint—actually, with its irregularly spaced
lines, the pattern bears a remarkably close resemblance to a DNA fingerprint.
(See Create a DNA Fingerprint.)

Every year a new ring of growth is added to a tree's trunk and branches, just
beneath the bark. The thickness of this ring depends on how much the tree grows
during the year: favorable growing conditions result in a fat ring, unfavorable
conditions, in a narrow ring. And because trees in a given area generally
experience the same growing conditions, those trees have similar tree-ring
patterns.

By starting with a living tree and working back, using wooden objects of
different ages, dendrochronologists have created continuous chains of tree
rings that stretch back hundreds, sometimes even thousands, of years. They then
use these tree-ring chronologies to date wooden objects, to determine a general
area in which the wood grew, to find out about past climates, and more.

This Hot Science lets you assemble your own tree-ring chronology. You'll start
with a tree-ring sample from a living tree and, using other samples, piece
together a pattern of rings that stretches back through time.